We are investigating approaches for utilizing recombinant DNA techniques for developing subunit vaccines for human respiratory syncytial virus (RSV). (An accompanying report describes efforts to use recombinant DNA technology to improve attenuated RSV vaccine strains.) We have been employing a panel of vaccinia recombinants expressing individually nine of the ten major RSV proteins to explore the roles of the different proteins in inducing antibodies and cytotoxic T cells (CTL), primarily in the BALB/c mouse. The issues of antibody induction, protective immunity and disease enhancement are described more thoroughly in an accompanying report (Murphy et al., accompanying report). A recombinant expressing the 22K or M2 protein (vac-M2) induced levels of CTL which were comparable to those induced by intranasal RSV infection. Vac-M2 induced significant levels of protective immunity in the absence of a detectable antibody response. This indicated that CTL alone can restrict the replication of RSV, a finding which had not been described previously for a virus which replicates in the respiratory tract. The use of adenovirus as a viral vector offers several potential advantages for RSV immunoprophylaxis. Previously, we described the construction of an adenovirus type 5 recombinant which expresses the F glycoprotein. The ad-F recombinant was highly immunogenic and protective in cotton rats (annual report, 1989, 1990) and mice (Murphy et al., accompanying report). However, ad-F was not effective in inducing RSV-specific serum antibodies in chimpanzees (Murphy et al., accompanying report). The possibility exists that the replacement of adenovirus sequences by the F gene altered the replication, expression or immunogenicity of the vector. Therefore, we are constructing a series of recombinants which contain insertion sites which are engineered to avoid the deletion of adenovirus sequences and to minimize alteration of adenoviral replication and gene expression.